Mold coating

ABSTRACT

A coating for the internal cavity of a mold for forming objects such as battery grids and the like including a perfluorocarbon film (e.g., - tetrafluoroethylene) having dispersed therein heatinsulating particles (e.g., - talc). The thickness of the film and the size and amount of the heat-insulating particles are maintained within defined limits to allow formation of complete grids with easy release from the mold.

United States Patent [1 1 Mao 1 1 Sept. 25, 1973 1 1 MOLD COATING [75] Inventor: George W. Mao, St. Paul, Minn.

[73] Assignee: Gould Inc., Mendota Heights, Minn.

[22] Filed: Aug. 9, 1971 [21] Appl. N0.: 170,067

[52] US. Cl 249/115, 117/5.3, 117/49,

117/75, 117/132 CF, 164/72 [51] Int. Cl. B29c 1/04, B28b 7/38 [58] Field of Search 117/5.3, 132 CF,

[56] References Cited UNITED STATES PATENTS 2,218,612 10/1940 Lockwood 249/115 X 3,279,936 10/1966 Forestek 1 1 249/115 X 3,080,258 3/1963 Davis 117/161 UF X 3,154,506 10/1964 Janssens 117/161 UF X 3,396,935 8/1968 Snyder l17/5.3 X

2,795,512 6/1957 Sheraat et a1 1l7/5.3

2,426,988 9/1947 Dean 1l7/5.3 3,230,056 1/1966 Arant et a1. 1l7/5.3 X 1,662,354 3/1928 Williams.... ll7/5.3 3,473,952 10/1969 McFadden..... 117/132 CF X 3,510,337 5/1970 Katzer et a1. 117/132 CF X Primary Examiner-William D. Martin Assistant ExaminerDennis C. Konopacki Att0rney--Wolfe, Hubbard, Leydig, Voit & Osann, Ltd.

[57] ABSTRACT 7 Claims, 3 Drawing Figures PATENTEDSEP25|975 761,047

SHEET 2 0F 2 INVENTOR GEORGE M M4 Arrys.

MOLD COATING This invention relates to forming objects and, more particularly, to a coating for molds such as are used in the forming of battery grids.

Battery grid casting is typically carried out on a semicontinuous basis; and, from an economic standpoint, it is desirable to employ a method of grid casting which allows formation of the grid with a minimum cycle time yet which controls the freezing or solidifying of the metal so that a complete grid is formed. Typically, the cycle time may vary from perhaps one grid per minute for a large industrial battery grid to to grids per minute for automotive batteries. Another requirement in casting operations of this type is that the formed grid must be able to be readily released from the mold surface.

Cast iron is the typical material used for such molds but does not adequately satisfy the requirements identified herein; for this reason, it is conventional practice to coat battery grid molds with various types of coatings in an attempt to provide the necessary mold surface characteristics for the grid casting operation. A widely used mold coating composition includes cork dust and clay suspended in water and sodium silicate. While such compositions function in an adequate manner, coatings of this type either disintegrate or are otherwise dislodged from the mold surface during continuous grid casting; and the coating has to be reapplied every 3 or 4 hours, which, of course, interrupts the grid casing operation and significantly reduces the production rate.

It is accordingly an object of the present invention to provide a coating for a mold which is characterized by an extended operating life. A related and more specific object lies in the .provision of a method of casting battery grids which enhances the production rate by minimizing the need for downtime.

Another object provides a coated mold with desirable flow properties across the mold surface for the fluid being employed and, also, which provides desirable release of the formed grid from the mold after the completion of the casting cycle.

Yet another object of the present invention is to provide a battery grid mold coating having heat-insulating properties optimally coordinating the freezing and s0- lidifying of the molten metal or alloy with the completeness of the grids formed so as to yield a minimum cycle time.

A still further object lies in the provision of a mold coating for battery grid casting which allows the displaced air to readily escape from the mold cavity during casting.

Other objects and advantages of the present inven tion will become apparent as the description proceeds, taken in conjunction with the accompanying drawings in which:

FIG. 1 is a plan view of one section of a battery grid mold of the top pour type having thereon the coating of the present invention and showing the gate area through which the casting metal or alloy flows and the grid cavity;

FIG. 2 is a cross-sectional view of the mold shown in FIG. 1 and schematically further showing the coating of the present invention, the size of the coating being exaggerated for illustrative purposes; and

FIG. 3 is a photomicrograph of a portion of a mold surface coated in accordance with the present invention and illustrating the surface roughness which allows the air to escape during filling of a mold cavity.

While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof have been described in detail herein. It should beunderstood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. For example, while the present invention may be advantageously employed in battery grid casting, it should be appreciated that the invention is equally applicable to the casting of other articles of manufacture wherein the casting operation is functionally similar to that of battery grid casting. Similarly, the present invention may also be employed in the extrusion of plastics, e.g., in injection molding operations.

Briefly, the present invention provides a mold having an adherent continuous perfluorocarbon resin film, preferably of relative uniform thickness thereon with a specified portion of heat-insulating particles dispersed within the film. The amount and size of the heatinsulating particles are controlled within specific limits to provide a mold surface having the desired heatinsulating properties as well as a surface with a controlled roughness, which allows the displaced air to escape and yet does not adversely affect beyond acceptable limits the surface of the cast grid. The invention is particularly useful in relative low temperature range grid casting operations. While such operations are typically carried out in the range of from 700 to 900F. depending upon the alloy or other molten material being cast, it has been unexpectedly found that perfluorocarbon resins such as polytetrafiuoroethylene may be employed to provide a long-lasting mold coating even though such resins are generally not recommended for usage at temperatures above about 500F. and are further considered as unstable at temperatures above about 600F.

In applying the perfluorocarbon resin, heat-insulating particlecoating of the present invention to the mold, it is desirable to first clean the mold surface. While perhaps not essential, cleaning improves the adhesion of the coating to the mold surface, typically cast iron. The surface may thus be cleaned with suitable solvents and degreasers which are well known. However, it has been found to be advantageous to carry out the cleaning step by preheating the mold to a temperature sufficient so that the surface is free of oils, grease and other impurities. This may be accomplished by heating the mold surface to a temperature of about 725 to 800F. and holding for a period of about 5 minutes or perhaps even more.

After allowing the temperature of the mold to reach ambient conditions, another preparatory step that may be carried out to also improve adhesion of the coating to the surface involves roughening the surface by mechanical or chemical means. Such techniques are well known and it has been found desirable to accomplish the surface roughening operation by sandblasting the mold surface to a smooth matte finish, e.g., by using 60 to grit size silica sand. After grit blasting, the grit should be blown off to insure that no particles stick to the surface of the mold, and the sandblasted surface may then be cleaned using a solvent such as trichloroethylene.

At this point, and in accordance with the present invention, a perfluorocarbon resin as defined herein is combined with specified amounts of heat-insulating particles to provide a mixture which may be applied to the mold surface to form a coating with desirable properties for battery grid casting applications. For the perfluorocarbon components, it is preferred to employ a polytetrafluoroethylene resin since such resins have particularly advantageous properties for this application. In addition, however, other suitable perfluorocarbons include mixtures of polytetrafluoroethylene and polymonochlorotrifluoroethylene, polymonochlorotrifluoroethylene, copolymers of tetrafluoroethylene with hexafluoropropylene, mixtures of polytetrafluoroethylene and polytetrafluoropropylene and equivalents thereof. To be useful, the perfluorocarbon component must be sufficiently non-adherent so that the formed object, such as the grid, may be readily separated from the mold and must be inert with respect to the molten fluid. Further, it should have sufficient stability so as not to be significantly affected by the molding temperatures being used.

In keeping with the invention, the particle component of the coating must be sized to provide a surface roughness which allows the air in the mold cavity to escape as the casting metal or alloy flows therein so that a substantially complete grid is formed yet not substantially impair the surface texture of the formed grid. In addition, the material used for the particles must possess heat-insulating properties adequate to keep the fluid molten until the mold cavity is filled. Still further, the particle material must be chemically inert with respect to the perfluorocarbon component and be capable of withstanding the cure temperatures required for such materials, as will be hereinafter described. Talc (hydrated magnesium silicate) and cork dust are preferred due to their relatively superior heat-insulating properties and other desirable properties. Other examples of materials that may perhaps be used include silica, alumina and diatomaceous earth. A particle size of about to 80 microns should be utilized to provide a uniform surface roughness essential to allowing the casting metal to fill the cavity while also imparting satisfactory characteristics as regards the application of the coating to the mold and the life of the coating.

The perfluorocarbon and the heat-insulating particle components should be combined in a manner that will achieve the shortest casting cycle time consistent with allowing the metal or alloy being cast a sufficient time to fill the cavity before freezing or solidifying. It is desirable to keep the thickness of the coating to a minimum, consistent with its functional requirements. Accordingly, to achieve such objectives, the thickness of the coating should be within the range of from about 1.5 to 3.5 mils and comprise, by volume, from about 10 to 40 percent of heat-insulating particles with the remainder being the perfluorocarbon component. The amount of the heat-insulating particles used and, to some extent, the thickness of the film is dependent upon the thickness of the object being formed and its size (i.e., the distance the molten fluid has to travel). For example, formation of a conventionally sized industrial battery grid having a thickness of about 0.2 inch can employ about 10 percent, by volume, particles while an automotive battery grid of conventional size and with a thickness of about 0.06 inch may require about 40 percent of the heatinsulating particles.

In keeping with the present invention, it is desirable to apply the coating to the mold surface by spraying. Techniques for spraying perfluorocarbon resin slurries are well known and may be adopted to apply the coating of this invention. Typically, the heat-insulating particles may be added to a perfluorocarbon solution; and the resulting solution mixed to provide uniform dispersion of the heat-insulating particles therein. The mixture or solution can be then strained through a stainless steel screen of, for example, mesh to insure that no coagulated perfluorocarbon-heat-insulating particles are transferred to the spraying apparatus. Conventional suction (airless) or pressure (compressed air) spraying equipment may be used, and such apparatus is commercially available. The spraying should be carried out in such a fashion as to provide a coating on the mold of relatively uniform thickness. Following spraying, the coating should be allowed to airset for a few mintues (e.g. up to about 5 minutes or so) to improve adhesion and then can be charged to a suitable furnace or other means where the coating is dryed for about 30 minutes at intermediate temperatures, i.e. 200F. Curing may then be effected by subjecting the coating to temperatures and dwell times which are adequate to cure the perfluorocarbon component. Any conventional furnace such as a direct fired gas, indirect fired gas or electric resistance can be used. The cure times and temperatures are known; and, for example, a suitable polytetrafluoroethylene coating can be cured by exposing the coating to a temperature of 450F. for at least about 20 minutes.

Referring now to the drawings, FIGS. 1 and 2 show a typical grid casting mold having adhered thereto the coating of the present invention. As illustrated, there is provided a stationary mold section 10 of the top pour, double-cavity type which comprises a gate 12 through which the molten metal or alloy being cast is passed. The gate 12 feeds cavities 14, 14. Two of such stationary mold sections are fitted together during casting to define the cavities therebetween and are then parted following solidification of the material being cast so that the formed grids from the cavities may be removed. The coating, indicated generally at 16, comprises a continuous film 18 of the perfluorocarbon component with heat-insulating particles 20 uniformly dispersed therein.

As shown in FIG. 3, which is a photograph of a portion of a mold surface coated in accordance with the present invention at 50 amplification, there is provided a controlled, relatively uniform surface roughness which allows the air to escape as the mold cavity fills. Quite obviously, it is unnecessary for the surface roughness caused by the particles to be absolutely uniform; but the heat-insulating particles should be sufficiently uniformly dispersed in the film to avoid hot spots" in the mold which could create non-uniform solidifying rates and to avoid development of significantly nonuniform air displacement.

In accordance with one aspect of the present invention, the perfluorocarbon, heat-insulating particle coating is applied only to the mold surface and the cavity areas and is not applied to the gate region. In this fashion, the non-insulated gate area functions to cool the temperature of the molten metal or alloy being cast so that the temperature of such material is as low as possible when it contacts the coating. This not only serves to increase the life of the coating but also functions to minimize the casting cycle time.

While the coating of the present invention has been herein described as a single layer, it should be appreciated that the use of multiple layers is also within the scope of the present invention. Thus, it may be desirable to employ a primer coating of the perfluorocarbon resin, which has been treated, as is known, to improve the adhesion characteristics of such coating to the cast iron mold. The second layer may then comprise an enamel coating, which readily adheres to the primer coating. In this embodiment, the primer coating can suitably have a thickness in the range of from about 0.5 to about 1 mil and the enamel coating can have a thickness of about 1.5 to 2.5 mils. Each of the layers should have dispersed therein the heat-insulating particles, and the amount in the primer layer should vary from about 5 to about 30 percent by volume and in the enamel layer from about to 40 percent by volume.

The following examples are intended to be merely illustrative of the use of the present invention in casting battery grids and are not in limitation thereof.

EXAMPLE 1 A cast iron mold suitable for forming battery grids having a size of 5 2 inches X 5 inches 0.064 (length width X thickness) was treated by cleaning the surface by preheating the mold in an electric resistance furnace at 750F. for minutes. The mold was then allowed to cool to ambient conditions, the surface sandblasted to a smooth matte finish using 60 to 80 grit size silica sand and cleaned using a trichloroethylene solvent.

Nine percent by volume talc having an average size of about microns throug l 1 a -2 0 'l:yl e r rnesh sc r e e n and using the fraction collected on a 400 mesh screen was dispersed in a TFE primer green No. 850-204" polytetrafluoroethylene resin solution (E. l. Du Pont de Nemours & Co., Wilmington, Delaware, 39 percent by volume resin solids) after the solution was heated to about 70 to 80F. Uniform dispersion was obtained by placing the solution into a container and mixing for about 15 minutes with a conventional driven roolfThe mixed solution was then sprayed directly onto the mold surface by using compressed air spraying equipment (DeVilbiss TSA No. 502 gun). The coating was allowed to airset for several minutes and was then cured at 650F. for about 2 hours in an electric resistance furnace. The thickness of the resultant coating was about 0.5 mils.

Thirty percent by volume talc having an average size of about 60 microns (obtained by passing the talc through a 200 mesh Tyler screen) was dispersed in a TFE enamel black and clear finish No. 851-245 polytetrafiuoroethylene resin solution (E. I. Du Pont de Nemours, Wilmington, Delaware, 41 percent by volume resin solids), with uniform dispersion being obtained by as previously described. The enamel coating was sprayed on the primer coating using the hereinidentified spraying equipment. After airset and curing by heating in an electric resistance furnace at 750F. for 30 minutes, a 2.5 mil film resulted. v

The thus-coated mold was used to form grids by passing a molten lead-antimony alloy (about 4 fa-7 percent by weight antimony) at a temperature which varied between 700-850F. into the cavity. The cast grids were complete in size and had a satisfactory surface appearance. The mold was used to cast grids for about a 24 hour period, and there was no evidence of the mold coat peeling.

EXAMPLE 2 A cast iron mold suitable for forming battery grids having a size of 10 inches X 6 inches X 0.18 inches was prepared by degreasing with a trichloro-ethylene solvent, the surface sandblasted with to 150 size aluminum oxide grit, rinsed to insure removal of dust and heated as in Example 1 at 750F. for about 15 minutes.

A recipe of 210ml. of Teflon S 953- polytetrafluoroethylene resin solution (E. I. Du Pont de Nemours & Co., Wilmington, Delware, about 33 percent by volume resin solids), 7 grams of cork flour and 50 ml. of a conventional thinner (T-8748", E. I. Du Pont de Nemours & Co., Wilmington, Delaware) were combined in solution and mixed thoroughly for about 5 minutes at the low speed level of a conventional Lightnin mixer. The particle size distribution (Tyler) of the cork flour was as follows: +80 mesh 0.5 percent; 80+140 1.4 percent; l40+200 36.5 percent; -200+325 49.0 percent and 325 12.5 percent. The apparent density of the recipe was about 0.7 gms/in.

The solution was sprayed onto the mold surface with conventional compressed air spraying equipment (DeVilbiss TGA No. 502 gun), and the resulting coating cured by air drying or setting for about 10 minutes followed by placing in an electric resistance furnace preheated to 600F. until the metal temperature inside the mold block reaches 400F. and is followed by 10 minutes at an air temperature of 600F. to provide a cured film with a thickness of about 1.5 mils.

The thus-coated mold was used to form grids from the lead-antimony alloy described in Example 1. The cast grids were complete in size and had a satisfactory surface appearance. The mold was used to cast grids for about a 24 hour period, and there was no evidence of the mold coat peeling.

Thus, as has been seen, the present invention provides a coating for a battery grid mold which consists of heat-insulating particles dispersed in a perfluorocarbon resin film. When combined as set forth herein, the life of the coating is relatively long and the heatinsulating properties are optimally coordinated with the freezing and solidifying of the molten metal or alloy so as to yield a minimum cycle time with complete for mation sizewise of the grids. The formed grids may be easily separated from the mold following the casting operation.

I claim as my invention:

1. In a mold for forming objects from a molten fluid having mating sections defining an internal cavity conforming to the object shape and a gate area to receive and channel the molten fluid to the internal cavity, the mating sections being capable of parting to allow the formed objects to be removed from the cavity, the improvement which comprisesa perfluorocarbon coating of from about 1.5 to 3.5 mils in thickness covering at least the internal cavity and having dispersed therein from about 10 to 40 percent, by volume, of heatinsulating particles with a size of from about 10 to 80 microns, the heat-insulating property of the particles being sufficient to delay solidification of the molten fluid until the fluid has at least substantially filled the cavity and the heat-insulating particles being sized to provide the coating surface with a roughness allowing the air in the internal cavity to escape as the molten fluid passes therethrough yet not substantially impairing the surface of the object being formed.

2. The coated mold of claim 1 wherein the perfluorocarbon is polytetrafluoroethylene.

3. The coated mold of claim 1 wherein the heatinsulating particles are formed of a member selected from the group consisting of talc, silica, alumina, diatomaceous earth and cork dust.

4. The coated mold of claim 3 wherein the heatinsulating particles are talc.

5. The coated mold of claim 3 wherein the heatinsulating particles are cork dust.

6. The coated mold of claim 1 wherein the heatinsulating particles are present in an amount of at least about 25 percent, by volume.

7. In a mold for forming objects from a molten fluid having mating sections defining an internal cavity conforming to the object shape and a gate area to receive and channel the molten fluid to the internal cavity, the

mating sections being capable of parting to allow the formed object to be removed from the cavity, the improvement which comprises a first perfluorocarbon coating of from about 0.5 to about 1 mil in thickness covering at least the internal cavity and having dispersed therein from about 5 to 30% by volume of heatinsulating particles with a size of from about lOto microns, a second perfluorocarbon coating of about 1.5 to 2.5 mils covering the first coating and having dispersed therein from about 10 to about 10% by volume of heat-insulating particles with a size of about 10 to 80 microns, the heat-insulating property of the particles of the two coatings being sufficient to delay solidification of the molten fluid until the fluid has at least substantially filled the cavity and the heat insulating particles being sized to provide the surface of the second coating with a roughness allowing the air in the internal cavity to escape as the molten fluid passes therethrough yet not substantially impairing the surface of the object being formed. 

1. In a mold for forming objects from a molten fluid having mating sections defining an internal cavity conforming to the object shape and a gate area to receive and channel the molten fluid to the internal cavity, the mating sections being capable of parting to allow the formed objects to be removed from the cavity, the improvement which comprises a perfluorocarbon coating of from about 1.5 to 3.5 mils in thickness covering at least the internal cavity and having dispersed therein from about 10 to 40 percent, by volume, of heat-insulating particles with a size of from about 10 to 80 microns, the heat-insulating property of the particles being sufficient to delay solidification of the molten fluid until the fluid has at least substantially filled the cavity and the heat-insulating particles being sized to provide the coating surface with a roughness allowing the air in the internal cavity to escape as the molten fluid passes therethrough yet not substantially impairing the surface of the object being formed.
 2. The coated mold of claim 1 wherein the perfluorocarbon is polytetrafluoroethylene.
 3. The coated mold of claim 1 wherein the heat-insulating particles are formed of a member selected from the group consisting of talc, silica, alumina, diatomaceous earth and cork dust.
 4. The coated mold of claim 3 wherein the heat-insulating particles are talc.
 5. The coated mold of claim 3 wherein the heat-insulating particles are cork dust.
 6. The coated mold of claim 1 wherein the heat-insulating particles are present in an amount of at least about 25 percent, by volume.
 7. In a mold for forming objects from a molten fluid having mating sections defining an internal cavity conforming to the object shape and a gate area to receive and channel the molten fluid to the internal cavity, the mating sections being capable of parting to allow the formed object to be removed from the cavity, the improvement which comprises a first perfluorocarbon coating of from about 0.5 to about 1 mil in thickness covering at least the internal cavity and having dispersed therein from about 5 to 30% by volume of heat-inSulating particles with a size of from about 10 to 80 microns, a second perfluorocarbon coating of about 1.5 to 2.5 mils covering the first coating and having dispersed therein from about 10 to about 10% by volume of heat-insulating particles with a size of about 10 to 80 microns, the heat-insulating property of the particles of the two coatings being sufficient to delay solidification of the molten fluid until the fluid has at least substantially filled the cavity and the heat insulating particles being sized to provide the surface of the second coating with a roughness allowing the air in the internal cavity to escape as the molten fluid passes therethrough yet not substantially impairing the surface of the object being formed. 